Atoms Rearranged in Reactions - Middle School Physical Science

1. Identify whether the equation $2H_2 + O_2 \rightarrow 2H_2O$ is balanced.

   Balanced. 4 H atoms and 2 O atoms on each side confirms balance.

2. What do subscripts represent in a chemical formula model?

   The number of each type of atom in one molecule. Small numbers indicate atom count within a single molecule.

3. What does it mean for a chemical equation to be balanced?

   Each element has equal atom counts on both sides. Ensures conservation of mass by matching atom counts.

4. What is the main idea about atoms in a chemical reaction (created, destroyed, or rearranged)?

   Atoms are rearranged, not created or destroyed. Conservation of mass means atoms only change partners, not quantity.

5. Which part of a chemical formula changes atom count: a coefficient or a subscript?

   Coefficient changes total count; subscript changes per molecule. Coefficients multiply entire formulas; subscripts define composition.

6. Identify whether this equation is balanced: $2H_2 + O_2 \rightarrow 2H_2O$.

   Balanced. 4 H atoms and 2 O atoms on each side.

7. Which change balances $Fe + O_2 \rightarrow Fe_2O_3$: add coefficient $2$ before $Fe_2O_3$ or before $O_2$?

   Add $2$ before $Fe_2O_3$. This gives $4Fe + 3O_2 → 2Fe_2O_3$, balancing Fe and O atoms.

8. What does it mean if a chemical equation is balanced?

   Each element has the same number of atoms on both sides. Conservation of atoms is satisfied when counts match.

9. What is a coefficient in a chemical equation used to represent in particle models?

   The number of molecules or formula units. Tells how many of each molecule participate in the reaction.

10. Which action is allowed when balancing equations: changing coefficients or changing subscripts?

    Changing coefficients only. Subscripts define the compound; changing them creates different substances.

11. Identify the total number of N atoms in $4NH_3$.

    $4$ N atoms. Each $NH_3$ has $1$ N atom: $4 \times 1 = 4$ total.

12. Which option shows conservation of atoms: breaking bonds, creating new atoms, or destroying atoms?

    Breaking bonds (and forming new bonds). Atoms separate and recombine without being created or destroyed.

13. What is conserved in a chemical reaction: atoms, mass, or both?

    Both atoms and total mass are conserved. The law of conservation applies to both matter and individual atoms.

14. What coefficient balances $\mathrm{\_\,H_2 + O_2 \rightarrow 2H_2O}$ for hydrogen atoms?

    $2$. This gives 4 H atoms on the reactant side to match products.

15. Which option is balanced: A) $H_2 + O_2 \rightarrow H_2O$ or B) $2H_2 + O_2 \rightarrow 2H_2O$?

    B) $2H_2 + O_2 \rightarrow 2H_2O$. Option A has unequal O atoms (2 vs 1).

16. Identify the coefficient needed to balance oxygen: $2H_2 + O_2 \rightarrow \square H_2O$.

    2. 2 water molecules contain 2 O atoms total.

17. What is the key modeling rule for atoms when building reactant and product particle diagrams?

    Do not add or remove atoms; only rearrange them. Ensures conservation of mass in particle diagrams.

18. What do the coefficients in a chemical equation represent in particle (atom/molecule) models?

    The number of molecules or formula units present. Coefficients show how many of each particle type react or form.

19. Identify whether $H_2 + O_2 \rightarrow H_2O$ is balanced for oxygen atoms.

    No; reactants have $2$ O atoms and products have $1$. Unbalanced because O atoms differ between sides.

20. Which statement best describes what chemical bonds do during a reaction?

    Bonds break and new bonds form. Atoms reconnect in new arrangements during reactions.

21. What does a balanced chemical equation show about the number of each type of atom before and after?

    Each element has the same atom count on both sides. Balancing ensures atoms are conserved, not created or destroyed.

22. What does the law of conservation of mass state for a closed chemical reaction system?

    Total mass of reactants equals total mass of products. Mass is conserved because atoms are neither created nor destroyed.

23. Find and correct the error: "Atoms are created when new products form."

    Correct: Atoms are rearranged; none are created or destroyed. Conservation of mass prohibits creating atoms.

24. Find the missing coefficient to balance $2Al + \_\_Cl_2 \rightarrow 2AlCl_3$.

    $3$. 3 $Cl_2$ provides 6 Cl atoms to match 2 $AlCl_3$ (2×3Cl).

25. What is the definition of a balanced chemical equation in terms of atom counts?

    Each element has equal atom counts on both sides. Conservation of mass requires matching atom inventories.

26. Which option correctly counts oxygen atoms in $3CO_2$ in a particle model?

    $6$ oxygen atoms. Each $CO_2$ has $2$ O atoms; $3×2=6$ total.

27. How many oxygen atoms are on the reactant side of $2CO + O_2 \rightarrow 2CO_2$?

    $4$ oxygen atoms. 2 O in $2CO$ plus 2 O in $O_2$ equals 4.

28. Identify the number of H atoms on the product side of $CH_4 + 2O_2 \rightarrow CO_2 + 2H_2O$.

    4 H atoms. $2H_2O$ means 2 molecules × 2 H atoms each.

29. Identify the number of O atoms in $2H_2 + O_2 \rightarrow 2H_2O$ on the product side.

    2 O atoms. $2H_2O$ means 2 molecules × 1 O atom each.

30. Which part of a chemical equation lists the products?

    The right side of the arrow. New substances formed appear after the reaction arrow.

31. Identify the coefficient needed: $C_3H_8 + 5O_2 \rightarrow 3CO_2 + \_\_\,H_2O$.

    $4$. 8 H atoms on right (from $C_3H_8$) form 4 water molecules.

32. What principle states that atoms are not created or destroyed in a chemical reaction?

    Law of Conservation of Mass (matter). Mass cannot be created or destroyed, only transformed.

33. What is the only change you are allowed to make to balance an equation?

    Change coefficients, not subscripts in formulas. Subscripts define the compound; only coefficients can adjust.

34. Identify the correct statement for a closed system reaction model: total mass before vs. after reaction.

    Total mass before equals total mass after. Conservation of mass means no mass is lost or gained.

35. Choose the correct coefficient to balance $H_2 + O_2 \rightarrow \_\_H_2O$ using atom conservation.

    $2$ (so $H_2 + O_2 \rightarrow 2H_2O$ is closer to balanced). Need $2H_2O$ to balance: $2$ H atoms + $2$ O atoms → $4$ H + $2$ O.

36. Which part of a chemical formula changes atom counts: coefficients or subscripts?

    Coefficients change atom counts; subscripts do not change. Subscripts define the molecule; coefficients count molecules.

37. Identify what is conserved in a balanced equation model: number of molecules or number of atoms?

    Number of atoms of each element (not number of molecules). Molecules can change in number, but atom counts must balance.

38. Identify the meaning of the arrow $(\rightarrow)$ in a chemical equation.

    Yields; reactants form products. The arrow shows the direction of the reaction.

39. What statement best describes what happens to atoms during a chemical reaction?

    Atoms are rearranged; none are created or destroyed. Bonds break and reform, but atoms themselves remain unchanged.

40. Find the missing coefficient to balance $2Na + Cl_2 \rightarrow \_\_NaCl$.

    $2$. $2Na$ atoms need $2NaCl$ to balance sodium atoms.

41. What does it mean to say atoms are conserved in a chemical reaction?

    Atoms are rearranged; none are created or destroyed. Same atoms exist before and after, just bonded differently.

42. What is the definition of a reactant in a chemical equation?

    A starting substance that is changed in the reaction. Reactants appear on the left side of the equation arrow.

43. What is the key modeling evidence that mass is conserved when a gas is produced?

    Mass stays constant only if the system is closed. Open systems lose gas mass; closed systems retain all products.

44. Identify the total number of O atoms in $3CO_2$.

    6 O atoms. 3 molecules × 2 O atoms per molecule.

45. How many hydrogen atoms are in $\mathrm{2NH_3}$?

    $6$ hydrogen atoms. Coefficient 2 × subscript 3 = 6 hydrogen atoms.

46. Which statement best describes bonds in a chemical reaction model?

    Bonds break in reactants and form new bonds in products. Chemical reactions rearrange atoms by changing their connections.

47. Find the missing coefficient: $2Na + \_\_\,Cl_2 \rightarrow 2NaCl$.

    $1$. 2 Cl atoms needed on left to match 2 Cl on right.

48. What do subscripts in a chemical formula represent in particle models?

    The number of atoms of each element in one particle. Subscripts show atom count within a single molecule.

49. What coefficient makes this equation balanced: $\_\_ H_2 + \_\_ O_2 \rightarrow 2H_2O$ (give the $H_2$ coefficient)?

    $2$. Provides 4 H to match the 4 H in products.

50. Which term names the substances present before a chemical reaction begins?

    Reactants. Starting materials that undergo chemical change.

51. What does it mean to say atoms are conserved in a chemical reaction?

    Atoms are rearranged; none are created or destroyed. Bonds break and reform, but the same atoms remain.

52. What does a balanced chemical equation show about atom counts on both sides?

    Each element has equal atom counts on both sides. Conservation of mass requires identical atom counts.

53. Identify the conserved atom counts for $2H_2 + O_2 \rightarrow 2H_2O$: how many $O$ atoms?

    $2$ $O$ atoms on each side. $O_2$ has $2$ O atoms; $2H_2O$ has $2 \times 1 = 2$ O atoms.

54. What coefficient balances $\mathrm{H_2 + O_2 \rightarrow \_\,H_2O}$ for oxygen atoms?

    $2$. This gives 2 O atoms on the product side to match reactants.

55. In $\mathrm{CH_4 + 2O_2 \rightarrow CO_2 + 2H_2O}$, how many hydrogen atoms are on the product side?

    $4$ hydrogen atoms. From $2H_2O$: coefficient 2 × subscript 2 = 4 H atoms.

56. What happens to atoms during a chemical reaction: created, destroyed, or rearranged?

    Atoms are rearranged; they are not created or destroyed. Conservation of mass means atoms only change their bonding patterns.

57. Find the coefficient for $H_2$ that balances: $H_2 + O_2 \rightarrow 2H_2O$.

    Coefficient of $H_2$ is $2$. 4 H atoms in products require 2 $H_2$ molecules.

58. What coefficient makes this equation balanced: $\_\_ N_2 + 3H_2 \rightarrow 2NH_3$?

    $1$. Balances 2 N atoms on left with 2 N in products.

59. Which quantity must be conserved in any chemical reaction: mass or volume?

    Mass. Volume can change, but mass is always conserved in reactions.

60. Find the missing coefficient: $\_\_\,N_2 + 3H_2 \rightarrow 2NH_3$.

    $1$. 2 N atoms on right require 2 N atoms on left from $N_2$.

61. What is the role of a catalyst regarding whether atoms are created or destroyed?

    It speeds the reaction; it does not create or destroy atoms. Catalysts lower activation energy without being consumed.

62. Which is allowed when balancing equations: changing coefficients or changing subscripts?

    Changing coefficients (not subscripts). Subscripts define the substance; changing them creates different compounds.

63. What does a subscript in a chemical formula represent in particle models?

    The number of atoms of that element per molecule. Indicates atom ratio within a single molecule.

64. Which statement is correct for a closed system reaction: mass of reactants vs products?

    Mass of reactants equals mass of products. Conservation of mass principle in action.

65. Identify the number of $O$ atoms on the product side of $CH_4 + 2O_2 \rightarrow CO_2 + 2H_2O$.

    $4$. $CO_2$ has $2$ O atoms + $2H_2O$ has $2$ O atoms = $4$ total.

66. Identify the element with unequal atoms in $2CO + O_2 \rightarrow 2CO_2$.

    None; both C and O are equal. 2 C and 4 O on each side shows perfect balance.

67. Find the missing coefficient to conserve atoms: $\_\_ CH_4 + 2O_2 \rightarrow CO_2 + 2H_2O$.

    $1$. Balances 1 C and 4 H to match products.

68. Identify the number of hydrogen atoms in $3\text{NH}_3$.

    9 hydrogen atoms. 3 molecules × 3 hydrogen atoms per molecule = 9 total.

69. Identify the missing coefficient that balances $2Na + \_\_\_ Cl_2 \rightarrow 2NaCl$.

    $1$. Cl atoms: $1×2=2$ on left equals $2×1=2$ on right.

70. What is a subscript in a chemical formula used to represent in particle models?

    The number of atoms of each element in one particle. Shows atom count within a single molecule or formula unit.

71. What must be true about total mass when a chemical reaction occurs in a closed system?

    Total mass stays the same. No mass enters or leaves a closed system.

72. Identify the missing coefficient that balances $\_\_\_ H_2 + \_\_\_ O_2 \rightarrow 2H_2O$.

    $2$ for $H_2$ and $1$ for $O_2$. Balances H: $2×2=4$ and O: $1×2=2$ to match products.

73. Identify whether $Mg + O_2 \rightarrow MgO$ is balanced as written.

    No; O is not conserved (needs $2Mg + O_2 \rightarrow 2MgO$). Left has $2$ O atoms, right has $1$; unbalanced.

74. Which model best shows conservation: same number of each atom color before and after, or different?

    Same number of each atom color before and after. Visual models must show atom conservation like equations do.

75. Find the missing coefficient: $2H_2 + O_2 \rightarrow H_2O$ to conserve H and O atoms.

    $2$. Coefficient balances both H ($4$ each side) and O ($2$ each side).

76. Identify whether this equation is balanced: $\mathrm{2H_2 + O_2 \rightarrow 2H_2O}$.

    Balanced. Each element has equal atoms on both sides: H=4, O=2.

77. What is a reactant in a chemical reaction model?

    A starting substance that is changed during the reaction. Reactants appear on the left side of the equation.

78. Identify the atoms present in $CH_4$ and their counts in one molecule.

    $1$ C atom and $4$ H atoms. No subscript means 1 atom; subscript 4 means 4 atoms.

79. Which side has more C atoms in $CH_4 + 2O_2 \rightarrow CO_2 + 2H_2O$?

    Neither side; both have 1 C atom. $CH_4$ has 1 C; $CO_2$ has 1 C.

80. Which statement matches a particle model of a reaction: atoms disappear or atoms rearrange into new molecules?

    Atoms rearrange into new molecules. Conservation means atoms persist but reorganize.

81. What is the definition of a reactant in a chemical reaction model?

    Starting substances that undergo change. These are the initial substances before the reaction occurs.

82. What do subscripts represent in a chemical formula such as $H_2O$?

    The number of each type of atom in one molecule. Subscripts show atomic composition of compounds.

83. What is the key difference between a chemical change and a physical change?

    Chemical change forms new substances; physical does not. Chemical changes involve breaking and forming bonds.

84. Identify the meaning of the subscript in $O_2$ in a particle model.

    Two oxygen atoms bonded in each molecule. Subscript 2 shows atoms per molecule, not total molecules.

85. Identify the total oxygen atoms on the reactant side of $2H_2 + O_2 \rightarrow 2H_2O$.

    $2$ oxygen atoms. $O_2$ has 2 oxygen atoms per molecule.

86. Which part of a chemical equation represents the substances formed?

    Products (right side of the arrow). Shows what new substances exist after the reaction.

87. Which option correctly counts hydrogen atoms in $2H_2SO_4$ in a particle model?

    $4$ hydrogen atoms. Each $H_2SO_4$ has $2$ H atoms; $2×2=4$ total.

88. Which option best describes atoms in a closed-system chemical reaction model?

    All atoms remain in the system and are rearranged. No atoms enter or leave; they only reorganize.

89. Identify the conserved atom counts for $2H_2 + O_2 \rightarrow 2H_2O$: how many $H$ atoms?

    $4$ $H$ atoms on each side. $2H_2$ gives $2 \times 2 = 4$ H atoms; $2H_2O$ also has $2 \times 2 = 4$.

90. Identify the meaning of a coefficient in $2H_2O$ in a particle model.

    Two molecules (two formula units) of $H_2O$. The coefficient 2 multiplies the entire water molecule.

91. What do coefficients in a chemical equation represent in particle models?

    The number of molecules or formula units. Coefficients multiply the entire formula unit.

92. Which side has more O atoms in $2H_2 + O_2 \rightarrow 2H_2O$: reactants or products?

    Neither; O atoms are equal on both sides. Both sides have $2$ O atoms, showing the equation is balanced.

93. What do coefficients represent in a particle model of a reaction?

    The number of molecules (or formula units) present. Coefficients multiply the entire formula.

94. What is the key difference between a physical change and a chemical reaction in models?

    Chemical: bonds change; physical: substances stay the same. New substances form only in chemical reactions.

95. What do models show about bonds during a chemical reaction?

    Bonds break and new bonds form between the same atoms. Chemical reactions involve bond rearrangement, not atom creation.

96. What is a product in a chemical reaction model?

    A new substance formed by rearranging atoms. Products appear on the right side of the equation.

97. Identify whether this model is balanced: $CH_4 + 2O_2 \rightarrow CO_2 + 2H_2O$.

    Balanced. 1 C, 4 H, and 4 O atoms on each side.

98. Identify the missing coefficient that balances $\_\_\_ N_2 + 3H_2 \rightarrow 2NH_3$.

    $1$. N atoms: $1×2=2$ on left equals $2×1=2$ on right.

99. What must be true about the number of each element's atoms before and after a reaction?

    The number of atoms of each element stays the same. Conservation of atoms means equal counts before and after.

100. In $\mathrm{CH_4 + 2O_2 \rightarrow CO_2 + 2H_2O}$, how many oxygen atoms are on the reactant side?

     $4$ oxygen atoms. Coefficient 2 × subscript 2 = 4 oxygen atoms.

101. What do coefficients represent in a chemical equation model?

     The number of molecules or formula units present. Shows how many of each molecule participate in the reaction.

102. What does a particle model show about bonds during a chemical reaction?

     Bonds break and new bonds form between existing atoms. Atoms disconnect and reconnect in new arrangements.

103. What does a coefficient in a chemical equation represent at the particle level?

     The number of molecules (or formula units) present. Coefficients multiply all atoms in the formula that follows.

104. What is the difference between reactants and products in a chemical reaction model?

     Reactants are starting substances; products are new substances formed. The arrow shows transformation from starting to ending substances.

105. Identify the element not conserved in $Fe + O_2 \rightarrow Fe_2O_3$ as written.

     Fe and O are not conserved (equation is unbalanced). Left: 1 Fe, 2 O; right: 2 Fe, 3 O shows imbalance.

106. What is the main purpose of balancing a chemical equation in terms of atoms?

     To show equal numbers of each type of atom on both sides. Conservation requires the same atoms before and after reaction.

107. Identify whether this statement is correct: In reactions, atoms can disappear if gas escapes.

     Incorrect; atoms are conserved, only the system is open. Atoms never disappear; mass appears lost in open systems.

108. Identify the coefficient needed: $\_\_\,H_2 + O_2 \rightarrow 2H_2O$.

     $2$. 4 H atoms on right require 4 H atoms from $2H_2$ on left.

109. Identify the missing coefficient that balances $\_\_\_ CH_4 + 2O_2 \rightarrow CO_2 + 2H_2O$.

     $1$. C: $1×1=1$, H: $1×4=4$, O: $2×2=4$ all balance.

110. Which change is allowed in balancing equations: coefficients or subscripts?

     Coefficients only. Changing subscripts would create different substances.

111. Choose the correct claim for a model of $\text{CH}_4 + 2\text{O}_2 \rightarrow \text{CO}_2 + 2\text{H}_2\text{O}$.

     Counts of C, H, and O atoms match on both sides. 1 C, 4 H, 4 O on left equals 1 C, 4 H, 4 O on right.

112. What law states that total mass is conserved because atoms are not created or destroyed in reactions?

     Law of Conservation of Mass. Mass remains constant because atoms are neither created nor destroyed.

113. Which side of a chemical equation lists the products: left or right?

     Right side. Products are the new substances formed after the arrow.

114. What does the law of conservation of mass state for a chemical reaction?

     Total mass of reactants equals total mass of products. Matter cannot be created or destroyed in chemical reactions.

115. What does the arrow ($\rightarrow$) in a chemical equation indicate?

     Reactants yield products. Shows transformation direction from starting to ending substances.

116. Identify the total hydrogen atoms on the product side of $2H_2 + O_2 \rightarrow 2H_2O$.

     $4$ hydrogen atoms. $2H_2O$ means 2 molecules × 2H per molecule.

117. Which statement is correct: chemical reactions create atoms or chemical reactions rearrange atoms?

     Chemical reactions rearrange atoms. Atoms cannot be created or destroyed, only rearranged.

118. What do coefficients in a chemical equation represent in particle (atom/molecule) models?

     The number of molecules or formula units present. Coefficients tell how many whole molecules react or form.

119. What does the law of conservation of mass state for a chemical reaction?

     Mass is conserved; reactant mass equals product mass. Matter cannot be created or destroyed in chemical reactions.

120. Which part of a chemical equation represents the starting substances?

     Reactants (left side of the arrow). Shows what substances exist before the reaction occurs.

121. Identify whether $2H_2 + O_2 \rightarrow 2H_2O$ conserves atoms for H and O.

     Yes; H: $4=4$, O: $2=2$. Both elements have equal counts on each side.

122. Find the missing coefficient to balance $\_\_N_2 + 3H_2 \rightarrow 2NH_3$.

     $1$. 1 $N_2$ provides 2 N atoms to match 2 $NH_3$ (2×1N).

123. What does a coefficient in a chemical equation represent in particle models?

     The number of molecules (or formula units). Shows how many whole units participate in the reaction.

124. Identify whether atoms are conserved in $CH_4 + 2O_2 \rightarrow CO_2 + 2H_2O$.

     Yes, it is balanced (atoms conserved). C: $1=1$, H: $4=4$, O: $4=4$ - all elements match.

125. What does a coefficient in a chemical equation represent?

     The number of molecules or formula units. Coefficients multiply the entire formula unit.

126. What does a subscript in a chemical formula represent at the particle level?

     The number of atoms of that element in one molecule. Subscripts show how atoms bond within a single molecule.

127. Which side of a chemical equation lists the reactants: left or right?

     Left side. Reactants are the starting materials before the arrow.

128. Which parts of a chemical equation can you change to balance it: subscripts or coefficients?

     Coefficients only; subscripts must not be changed. Changing subscripts would create different compounds.

129. How many oxygen atoms are in $\mathrm{3CO_2}$?

     $6$ oxygen atoms. Coefficient 3 × subscript 2 = 6 oxygen atoms.

130. Identify whether this equation is balanced: $H_2 + O_2 \rightarrow H_2O$.

     Not balanced. Left has 2 H and 2 O; right has 2 H and 1 O.

131. Identify whether this equation is balanced: $\mathrm{H_2 + O_2 \rightarrow H_2O}$.

     Not balanced. Left has 2 O atoms, right has 1 O atom.

132. What must be true about each element when a chemical equation is balanced?

     Each element has the same atom count on both sides. Balancing ensures conservation of mass for each element.

133. Which change is allowed when balancing: changing coefficients or changing subscripts in formulas?

     Change coefficients only; never change subscripts. Subscripts define compounds; coefficients balance equations.

134. Identify the balanced equation: $2H_2 + O_2 \rightarrow 2H_2O$ or $H_2 + O_2 \rightarrow H_2O$.

     $2H_2 + O_2 \rightarrow 2H_2O$. 4 H and 2 O on each side makes it balanced.

135. What does a subscript in a chemical formula represent?

     The number of atoms of that element in one particle. Subscripts show atom ratios within a molecule.

136. What is a useful rule for particle models to show conservation of atoms in reactions?

     Use the same count of each colored atom before and after. Conservation means atoms neither appear nor disappear.

137. What does it mean if an equation is balanced in terms of atom models?

     Each element has equal atom counts before and after. Balance means conservation of each element's total atom count.

138. Which statement best matches particle models: mass changes, or mass stays constant in a closed system?

     Mass stays constant in a closed system. Conservation of mass applies to all closed-system reactions.

139. Which side has more H atoms in $CH_4 + 2O_2 \rightarrow CO_2 + 2H_2O$: reactants or products?

     Neither; H atoms are equal on both sides. Both sides have $4$ H atoms, confirming balance.

140. What is the main purpose of balancing a chemical equation?

     To show equal numbers of each atom on both sides. This follows the law of conservation of mass.

141. What is the law stating that total mass stays constant during a chemical reaction in a closed system?

     Law of Conservation of Mass. Mass cannot be created or destroyed in chemical reactions.

142. Find the missing coefficient to balance: $\text{N}_2 + 3\text{H}_2 \rightarrow \; ?\; \text{NH}_3$.

     2 (use $2\text{NH}_3$). 2 N atoms in N₂ require 2 NH₃ molecules.

143. What does a particle model show about atoms during a chemical reaction?

     Atoms are conserved and only rearranged. Models show atoms regrouping, not appearing or vanishing.

144. What does it mean to say atoms are rearranged in a chemical reaction?

     Atoms change partners to form new substances. Bonds break and new bonds form between different atoms.

145. What is the main goal when balancing a chemical equation model?

     Equal atom counts for each element on both sides. Conservation of mass requires equal atoms before and after.

146. Identify the number of oxygen atoms in $2\text{CO}_2$.

     4 oxygen atoms. 2 molecules × 2 oxygen atoms per molecule = 4 total.

147. Find the missing coefficient to balance $\_\_N_2 + 3H_2 \rightarrow 2NH_3$.

     $1$. $2NH_3$ has $2$ N atoms, so need $1N_2$ on reactant side.

148. What do subscripts in a chemical formula represent in an atom model (for example, in $H_2O$)?

     The number of atoms of each element in one molecule. Subscripts indicate atomic composition within a single molecule.

149. What principle states that atoms are not created or destroyed in a chemical reaction?

     Law of conservation of mass. Mass remains constant because atoms are neither created nor destroyed.

150. What condition must be true for an equation to be balanced in terms of atoms?

     Each element has equal atom counts on both sides. This satisfies the Law of Conservation of Mass.

151. Identify the number of chlorine atoms in $4\text{NaCl}$.

     4 chlorine atoms. 4 formula units × 1 chlorine atom per unit = 4 total.

152. Which option best describes why mass is conserved: atoms disappear or atoms are rearranged?

     Atoms are rearranged. Conservation occurs because atoms persist through reactions.

153. Find the coefficient for $O_2$ that balances: $2H_2 + O_2 \rightarrow 2H_2O$.

     Coefficient of $O_2$ is $1$. 2 O atoms in products require 1 $O_2$ molecule.

154. Identify the total number of H atoms in $2NH_3$.

     $6$ H atoms. Each $NH_3$ has $3$ H atoms: $2 \times 3 = 6$ total.

155. Identify the total number of atoms in one molecule of $CO_2$.

     3 atoms. 1 carbon + 2 oxygen atoms.

156. What coefficient makes this equation balanced: $2Na + Cl_2 \rightarrow \_\_ NaCl$?

     $2$. Matches 2 Na and 2 Cl atoms on each side.

157. Find the coefficient for $O_2$ that balances: $2Mg + O_2 \rightarrow 2MgO$.

     Coefficient of $O_2$ is $1$. 2 O atoms in products require 1 $O_2$ molecule.

158. Identify the type of change where atoms are rearranged into new substances.

     Chemical change. New substances form when atoms rearrange their bonds.

159. Identify the type of change where atoms are not rearranged into new substances.

     Physical change. No new substances form; only state or appearance changes.

160. Find and correct the mistake: $\mathrm{NaCl \rightarrow Na + Cl}$ does not conserve atoms.

     Correct: $\mathrm{NaCl \rightarrow Na + Cl_2}$ or use $\mathrm{2NaCl \rightarrow 2Na + Cl_2}$. Chlorine exists as diatomic $Cl_2$, or balance with coefficients.

161. Find the atom count of O on the reactant side of $2KClO_3 \rightarrow 2KCl + 3O_2$.

     $6$ oxygen atoms. Each $KClO_3$ has $3$ O atoms; $2×3=6$ total.

162. Choose the correct statement for a model: breaking bonds and forming bonds changes what?

     It changes how atoms are connected, not the atoms themselves. Bonds determine molecular structure while atoms remain constant.

163. Identify the total atoms in $3CO_2$ using a particle-count model.

     $3$ C atoms and $6$ O atoms. Coefficient 3 gives 3 molecules × (1C + 2O) per molecule.

164. What does it mean to say atoms are rearranged in a chemical reaction?

     Atoms change partners to form new substances. Bonds break and atoms recombine into different molecules.

165. What changes during a chemical reaction: the types of atoms or their arrangement?

     Only the arrangement changes; the types of atoms stay the same. Atoms form new bonds but remain the same elements.

166. What do subscripts in a chemical formula represent in an atom model?

     The number of atoms of each element in one particle. Subscripts show atomic composition within a single molecule.

167. What model-based evidence shows mass is conserved in a closed-system reaction?

     Same number of each type of atom before and after. Equal atom counts prove conservation of mass.

168. What does it mean to balance an equation in terms of models of atoms?

     Adjust coefficients so each element has equal atoms on both sides. Conservation requires same atom totals before and after.

169. Identify what a correct before-and-after particle model must show for each element.

     Same count of each element's atoms before and after. Conservation means identical element inventories on both sides.

170. What law states that total mass stays constant during a chemical reaction in a closed system?

     Law of Conservation of Mass. Mass cannot be created or destroyed in chemical reactions.

171. Which term names the substances formed after a chemical reaction occurs?

     Products. New substances created from rearranged reactant atoms.

172. What must be true about each element when a chemical equation is balanced?

     Same number of each type of atom on both sides. Conservation of mass requires equal atom counts.

173. What is the definition of a product in a chemical equation?

     A new substance formed by the reaction. Products appear on the right side of the equation arrow.

174. Identify whether $2Na + Cl_2 \rightarrow 2NaCl$ conserves atoms for Na and Cl.

     Yes; Na: $2=2$, Cl: $2=2$. Both elements have equal counts on each side.

175. Identify what must be the same on both sides of a balanced chemical equation.

     The number of atoms of each element. Conservation requires equal atom counts to maintain mass balance.

176. What is the definition of a product in a chemical reaction model?

     New substances formed by the reaction. These result from rearranging reactant atoms.

177. Identify the number of H atoms in $2H_2 + O_2 \rightarrow 2H_2O$ on the reactant side.

     4 H atoms. $2H_2$ means 2 molecules × 2 atoms each.

178. Which part of a chemical equation lists the reactants?

     The left side of the arrow. Starting materials appear before the reaction arrow.

179. Which equation is balanced: $H_2 + O_2 \rightarrow H_2O$ or $2H_2 + O_2 \rightarrow 2H_2O$?

     $2H_2 + O_2 \rightarrow 2H_2O$. Second equation has equal H and O atoms on both sides.

180. How many hydrogen atoms are in $3H_2O$ in a particle model?

     $6$ hydrogen atoms. 3 molecules × 2 H atoms per molecule = 6.

181. What is the law that states atoms are not created or destroyed in a chemical reaction?

     Law of Conservation of Mass. States that matter cannot be created or destroyed in chemical reactions.

182. Find and correct the error: balancing by changing $H_2O$ to $H_2O_2$ to match oxygen atoms.

     Incorrect; change coefficients, not subscripts or formulas. Changing formulas creates different compounds entirely.

183. Which change balances an equation: changing coefficients or changing subscripts?

     Changing coefficients (not subscripts). Subscripts define the compound; only coefficients can be adjusted.

184. Find the missing coefficient to balance: $2\text{Na} + \text{Cl}_2 \rightarrow \; ?\; \text{NaCl}$.

     2 (use $2\text{NaCl}$). 2 Na atoms need 2 NaCl to balance with Cl₂.

185. Identify what a balanced equation demonstrates about atoms in reactants and products.

     Each element has the same atom count on both sides. This satisfies conservation of mass.

186. Which statement best describes a chemical reaction compared with a physical change?

     A chemical reaction forms new substances by rearranging atoms. Physical changes don't rearrange atoms into new compounds.

187. Which option best describes why mass may appear to change in an open system reaction?

     Matter (often gas) enters or leaves the system. Mass changes when substances cross system boundaries.

188. What is the correct statement about atoms in a chemical reaction: created, destroyed, or rearranged?

     Atoms are rearranged into new substances. Conservation of mass means atoms only change arrangement.

189. Identify whether the equation $H_2 + O_2 \rightarrow H_2O$ is balanced.

     Not balanced. Left has 2 H and 2 O; right has 2 H but only 1 O.

190. Which modeling check best shows atoms are conserved: matching colors, shapes, or total atom counts?

     Total atom counts for each element match on both sides. Verifies atoms are rearranged, not created/destroyed.

191. What does it mean to say atoms are rearranged during a chemical reaction?

     Atoms change bonding partners; the same atoms remain. Atoms form new bonds but remain the same elements.

Study these flashcards